Regulation of renal phosphate transport by FGF23 is mediated by FGFR1 and FGFR4

Fibroblast growth factor-21 potentiates the stimulatory effects of 1,25-dihydroxyvitamin D3 on transepithelial calcium transport and TRPV6 Ca2+ channel expression

Fibroblast growth factor (FGF)-21 is a salient liver-derived endocrine regulator for metabolism of glucose and triglyceride as well as bone remodeling. Previously, certain peptides in the FGF family have been shown to modulate calcium absorption across the intestinal epithelia. Since FGF21 receptor, i.e., FGF receptor-1, is abundantly expressed in the enterocytes, there was a possibility that FGF21 might exert direct actions on the intestine. Herein, a large-scale production of recombinant FGF21 at the multi-gram level was developed in order to minimize variations among various batches. In the oral glucose tolerance test, recombinant FGF21 was found to reduce plasma glucose levels in mice fed high-fat diet. A series of experiments applying radioactive tracer 45Ca in Ussing chamber showed that FGF21 potentiated the stimulatory effect of low-dose 1,25-dihydroxyvitamin D3 [10 nM 1,25(OH)2D3] on the transepithelial calcium transport across intestinal epithelium-like Caco-2 monolayer. FGF21 + 1,25(OH)2D3 also decreased transepithelial resistance, but had no effect on epithelial potential difference or short-circuit current. Furthermore, 1,25(OH)2D3 alone upregulated the Caco-2 mRNA expression of the major apical calcium channels, i.e., transient receptor potential vanilloid subfamily member 6 (TRPV6), which was further elevated by a combination of FGF21 and 1,25(OH)2D3, consistent with the upregulated TRPV6 protein expression in enterocytes of FGF21-treated mice. However, FGF21 was without effects on the mRNA expression of voltage-gated calcium channel 1.3, calbindin-D9k, plasma membrane Ca2+-ATPase 1b, claudin-12 or claudin-15. In conclusion, FGF21 did exert a direct action on the intestinal epithelial cells by potentiating the 1,25(OH)2D3-enhanced calcium transport, presumably through the upregulation of TRPV6 expression.

The role of fibroblast growth factor 23 in regulation of phosphate balance

Phosphate is essential for numerous biological processes, and serum levels are tightly regulated to accomplish these functions. The regulation of serum phosphate in a narrow physiological range is a well-orchestrated process and involves the gastrointestinal (GI) tract, bone, kidneys, and several hormones, namely, parathyroid hormone, fibroblast growth factor 23 (FGF23), and 1,25-dihydroxyvitamin D (1,25 Vitamin D). Although primarily synthesized in the bone, FGF23, an endocrine FGF, acts on the kidney to regulate phosphate and Vitamin D homeostasis by causing phosphaturia and reduced levels of 1,25 Vitamin D. Recent studies have highlighted the complex regulation of FGF23 including transcriptional and post-translational modification and kidney-bone cross talk. Understanding FGF23 biology has led to the identification of novel therapeutic agents to treat diseases that disrupt phosphate metabolism secondary to FGF23. The focus of this review is to provide an overview of phosphate homeostasis, FGF23 biology, and the role of FGF23 in phosphate balance.

Discovery of LHQ490 as a highly selective fibroblast growth factor receptor 2 (FGFR2) inhibitor

Fibroblast growth factor receptor 2 (FGFR2) represents an appealing therapeutic target for multiple cancers, yet no selective FGFR2 inhibitors have been approved for clinical use to date. Here, we report the discovery of a series of new selective, irreversible FGFR2 inhibitors. The representative compound LHQ490 potently inhibited FGFR2 kinase activity with an IC50 of 5.2 nM, and was >61-, >34-, and >293-fold selective against FGFR1, FGFR3, and FGFR4, respectively. LHQ490 also exhibited high selectivity in a panel of 416 kinases. Cell-based studies revealed that LHQ490 efficiently suppressed the proliferation of BaF3-FGFR2 cells with an IC50 value of 1.4 nM, and displayed >70- and >714-fold selectivity against BaF3-FGFR1 and the parental BaF3 cells, respectively. More importantly, LHQ490 potently suppressed the FGFR2 signaling pathways, selectively inhibited FGFR2-driven cancer cell proliferation, and induced apoptosis of FGFR2-driven cancer cells. Taken together, this study provides a potent and highly selective FGFR2 inhibitor for further development of FGFR2-targeted therapeutic agents.

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Safety Profile and Adverse Event Management for Futibatinib, An Irreversible FGFR1-4 Inhibitor: Pooled Safety Analysis of 469 Patients

PURPOSE

Futibatinib, a covalently-binding inhibitor of fibroblast growth factor receptor (FGFR)1-4 gained approval for the treatment of refractory, advanced intrahepatic cholangiocarcinoma (iCCA) harboring an FGFR2 fusion/other rearrangement. An integrated analysis was performed to evaluate safety and provide guidance on the management of futibatinib-associated adverse events (AEs) in patients with unresectable/metastatic tumors, including iCCA.

PATIENTS AND METHODS

Data from three global phase I or II studies of futibatinib (NCT02052778; JapicCTI-142552) were pooled. AEs were graded per NCI CTCAE v4.03, where applicable. Safety was analyzed for patients receiving any futibatinib starting dose (overall population) and in those receiving the approved starting dose of 20 mg once every day.

RESULTS

In total, 469 patients with one of 33 known tumor types were analyzed, including 318 patients who received futibatinib 20 mg every day. AEs of clinical interest (AECI; any grade/grade ≥3) in the overall population included hyperphosphatemia (82%/19%), nail disorders (27%/1%), hepatic AEs (27%/11%), stomatitis (19%/3%), palmar-plantar erythrodysesthesia syndrome (PPES; 13%/3%), rash (9%/0%), retinal disorders (8%/0%), and cataract (4%/1%). Median time to onset of grade ≥3 AECIs ranged from 9 days (hyperphosphatemia) to 125 days (cataract). Grade ≥3 hyperphosphatemia, hepatic AEs, PPES, and nail disorders resolved to grade ≤2 within a median of 7, 7, 8, and 28 days, respectively. Discontinuations due to treatment-related AEs were rare (2%), and no treatment-related deaths occurred. AE management included phosphate-lowering medication and dose adjustments.

CONCLUSIONS

Futibatinib showed a consistent and manageable safety profile across patients with various tumor types. AECIs were mostly reversible with appropriate clinical management.

Discovery of lirafugratinib (RLY-4008), a highly selective irreversible small-molecule inhibitor of FGFR2

Fibroblast growth factor receptor (FGFR) kinase inhibitors have been shown to be effective in the treatment of intrahepatic cholangiocarcinoma and other advanced solid tumors harboring FGFR2 alterations, but the toxicity of these drugs frequently leads to dose reduction or interruption of treatment such that maximum efficacy cannot be achieved. The most common adverse effects are hyperphosphatemia caused by FGFR1 inhibition and diarrhea due to FGFR4 inhibition, as current therapies are not selective among the FGFRs. Designing selective inhibitors has proved difficult with conventional approaches because the orthosteric sites of FGFR family members are observed to be highly similar in X-ray structures. In this study, aided by analysis of protein dynamics, we designed a selective, covalent FGFR2 inhibitor. In a key initial step, analysis of long-timescale molecular dynamics simulations of the FGFR1 and FGFR2 kinase domains allowed us to identify differential motion in their P-loops, which are located adjacent to the orthosteric site. Using this insight, we were able to design orthosteric binders that selectively and covalently engage the P-loop of FGFR2. Our drug discovery efforts culminated in the development of lirafugratinib (RLY-4008), a covalent inhibitor of FGFR2 that shows substantial selectivity over FGFR1 (~250-fold) and FGFR4 (~5,000-fold) in vitro, causes tumor regression in multiple FGFR2-altered human xenograft models, and was recently demonstrated to be efficacious in the clinic at doses that do not induce clinically significant hyperphosphatemia or diarrhea.

FGF23 and klotho at the intersection of kidney and cardiovascular disease

Cardiovascular disease is the leading cause of death in patients with chronic kidney disease (CKD). As CKD progresses, CKD-specific risk factors, such as disordered mineral homeostasis, amplify traditional cardiovascular risk factors. Fibroblast growth factor 23 (FGF23) regulates mineral homeostasis by activating complexes of FGF receptors and transmembrane klotho co-receptors. A soluble form of klotho also acts as a 'portable' FGF23 co-receptor in tissues that do not express klotho. In progressive CKD, rising circulating FGF23 levels in combination with decreasing kidney expression of klotho results in klotho-independent effects of FGF23 on the heart that promote left ventricular hypertrophy, heart failure, atrial fibrillation and death. Emerging data suggest that soluble klotho might mitigate some of these effects via several candidate mechanisms. More research is needed to investigate FGF23 excess and klotho deficiency in specific cardiovascular complications of CKD, but the pathophysiological primacy of FGF23 excess versus klotho deficiency might never be precisely resolved, given the entangled feedback loops that they share. Therefore, randomized trials should prioritize clinical practicality over scientific certainty by targeting disordered mineral homeostasis holistically in an effort to improve cardiovascular outcomes in patients with CKD.

RLY-4008, the First Highly Selective FGFR2 Inhibitor with Activity across FGFR2 Alterations and Resistance Mutations

Oncogenic activation of fibroblast growth factor receptor 2 (FGFR2) drives multiple cancers and represents a broad therapeutic opportunity, yet selective targeting of FGFR2 has not been achieved. Although the clinical efficacy of pan-FGFR inhibitors (pan-FGFRi) validates FGFR2 driver status in FGFR2 fusion-positive intrahepatic cholangiocarcinoma, their benefit is limited by incomplete target coverage due to FGFR1- and FGFR4-mediated toxicities (hyperphosphatemia and diarrhea, respectively) and the emergence of FGFR2 resistance mutations. RLY-4008 is a highly selective, irreversible FGFR2 inhibitor designed to overcome these limitations. In vitro, RLY-4008 demonstrates >250- and >5,000-fold selectivity over FGFR1 and FGFR4, respectively, and targets primary alterations and resistance mutations. In vivo, RLY-4008 induces regression in multiple xenograft models-including models with FGFR2 resistance mutations that drive clinical progression on current pan-FGFRi-while sparing FGFR1 and FGFR4. In early clinical testing, RLY-4008 induced responses without clinically significant off-isoform FGFR toxicities, confirming the broad therapeutic potential of selective FGFR2 targeting.

SIGNIFICANCE

Patients with FGFR2-driven cancers derive limited benefit from pan-FGFRi due to multiple FGFR1-4-mediated toxicities and acquired FGFR2 resistance mutations. RLY-4008 is a highly selective FGFR2 inhibitor that targets primary alterations and resistance mutations and induces tumor regression while sparing other FGFRs, suggesting it may have broad therapeutic potential. See related commentary by Tripathi et al., p. 1964. This article is featured in Selected Articles from This Issue, p. 1949.

Renal FGF23 signaling depends on redox protein Memo1 and promotes orthovanadate-sensitive protein phosphotyrosyl phosphatase activity

Memo1 deletion in mice causes premature aging and an unbalanced metabolism partially resembling Fgf23 and Klotho loss-of-function animals. We report a role for Memo's redox function in renal FGF23-Klotho signaling using mice with postnatally induced Memo deficiency in the whole body (cKO). Memo cKO mice showed impaired FGF23-driven renal ERK phosphorylation and transcriptional responses. FGF23 actions involved activation of oxidation-sensitive protein phosphotyrosyl phosphatases in the kidney. Redox proteomics revealed excessive thiols of Rho-GDP dissociation inhibitor 1 (Rho-GDI1) in Memo cKO, and we detected a functional interaction between Memo's redox function and oxidation at Rho-GDI1 Cys79. In isolated cellular systems, Rho-GDI1 did not directly affect FGF23-driven cell signaling, but we detected disturbed Rho-GDI1 dependent small Rho-GTPase protein abundance and activity in the kidney of Memo cKO mice. Collectively, this study reveals previously unknown layers in the regulation of renal FGF23 signaling and connects Memo with the network of small Rho-GTPases.

Fibroblast growth factor 23, klotho and heparin

PURPOSE OF REVIEW

Fibroblast growth factor (FGF) 23 is a bone-derived hormone that regulates phosphate and vitamin D metabolism by targeting the kidney. When highly elevated, such as in chronic kidney disease (CKD), FGF23 can also target the heart and induce pathologic remodeling. Here we discuss the mechanisms that underlie the physiologic and pathologic actions of FGF23, with focus on its FGF receptors (FGFR) and co-receptors.

RECENT FINDINGS

Klotho is a transmembrane protein that acts as an FGFR co-receptor for FGF23 on physiologic target cells. Klotho also exists as a circulating variant, and recent studies suggested that soluble klotho (sKL) can mediate FGF23 effects in cells that do not express klotho. Furthermore, it has been assumed that the actions of FGF23 do not require heparan sulfate (HS), a proteoglycan that acts as a co-receptor for other FGF isoforms. However, recent studies revealed that HS can be part of the FGF23:FGFR signaling complex and modulate FGF23-induced effects.

SUMMARY

sKL and HS have appeared as circulating FGFR co-receptors that modulate the actions of FGF23. Experimental studies suggest that sKL protects from and HS accelerates CKD-associated heart injury. However, the in vivo relevance of these findings is still speculative.

Current and emerging therapies for Achondroplasia: The dawn of precision medicine

Achondroplasia is a rare disease affecting bone growth and is caused by a missense mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. In the past few years, there were multiple experimental drugs entering into clinical trials for treating achondroplasia including vosoritide, the first precision medicine approved for this indication. This perspective presents the mechanism of action, benefit, and potential mechanistic limitation of the drugs currently being evaluated in clinical trials for achondroplasia. This article also discusses the potential impact of those drugs not only in increasing the growth of individuals living with achondroplasia but also in improving their quality of life.

Exploring the Structural and Functional Diversity among FGF Signals: A Comparative Study of Human, Mouse, and Xenopus FGF Ligands in Embryonic Development and Cancer Pathogenesis

Fibroblast growth factors (FGFs) encode a large family of growth factor proteins that activate several intracellular signaling pathways to control diverse physiological functions. The human genome encodes 22 FGFs that share a high sequence and structural homology with those of other vertebrates. FGFs orchestrate diverse biological functions by regulating cellular differentiation, proliferation, and migration. Dysregulated FGF signaling may contribute to several pathological conditions, including cancer. Notably, FGFs exhibit wide functional diversity among different vertebrates spatiotemporally. A comparative study of FGF receptor ligands and their diverse roles in vertebrates ranging from embryonic development to pathological conditions may expand our understanding of FGF. Moreover, targeting diverse FGF signals requires knowledge regarding their structural and functional heterogeneity among vertebrates. This study summarizes the current understanding of human FGF signals and correlates them with those in mouse and Xenopus models, thereby facilitating the identification of therapeutic targets for various human disorders.

Dietary vitamin D3 deprivation suppresses fibroblast growth factor 23 signals by reducing serum phosphorus levels in laying hens

The present study was carried out to evaluate the effect of dietary supplemental vitamin D₃ on fibroblast growth factor 23 (FGF23) signals as well as phosphorus homeostasis and metabolism in laying hens. Fourteen 40-week-old Hy-Line Brown layers were randomly assigned into 2 treatments: 1) vitamin D₃ restriction group (n = 7) fed 0 IU/kg vitamin D₃ diet, and 2) regular vitamin D₃ group (n = 7) fed 1,600 IU/kg vitamin D₃ diet. The study lasted for 21 d. Serum parameters, phosphorus and calcium excretion status, and tissue expressions of type II sodium-phosphate co-transporters (NPt2), FGF23 signals and vitamin D₃ metabolic regulators were determined. Hens fed the vitamin D₃ restricted diet had decreased serum phosphorus levels (by 31.3%, P = 0.028) when compared to those fed regular vitamin D₃ diet. In response to the decreased serum phosphorus, the vitamin D₃ restricted laying hens exhibited: 1) suppressed kidney expressions of 25-hydroxyvitamin D 1-α-hydroxylase (CYP27B1, by 52.8%, P = 0.036) and 1,25-dihydroxyvitamin D 24-hydroxylase (CYP24A1, by 99.4%, P = 0.032); 2) suppressed serum levels of FGF23 (by 14.6%, P = 0.048) and increased serum alkaline phosphatase level (by 414.1%, P = 0.012); 3) decreased calvaria mRNA expressions of fibroblast growth factor receptors (FGFR1, by 85.2%, P = 0.003, FGFR2, by 89.4%, P = 0.014, FGFR3, by 88.8%, P = 0.017, FGFR4, by 89.6%, P = 0.030); 4) decreased kidney mRNA expressions of FGFR1 (by 65.5%, P = 0.021), FGFR4 (by 66.0%, P = 0.050) and KLOTHO (by 68.8%, P = 0.038); 5) decreased kidney protein expression of type 2a sodium-phosphorus co-transporters (by 54.3%, P = 0.039); and 6) increased percent excreta calcium (by 26.9%, P = 0.002). In conclusion, the deprivation of dietary vitamin D₃ decreased FGF23 signals in laying hens by reducing serum FGF23 level and suppressing calvaria and kidney mRNA expressions of FGF23 receptors.

Changes in Serum Intact Fibroblast Growth Factor 23 Concentrations From Midlife to Late Life and Their Predictors in the Community: The ARIC Study

Objective

To investigate longitudinal changes in the blood concentration of fibroblast growth factor 23 (FGF23) from midlife to late life and their major predictors in the general population.

Patients and Methods

In 14,444 participants of the Atherosclerosis Risk in Communities Study, we analyzed the association of 31,095 measurements of serum intact FGF23 with age using data from 3 visits (visit 2 [N=13,460; mean age, 57 years]; visit 3 [N=12,323; mean age, 60 years]; and visit 5 [N=6122; mean age, 76 years]) and a linear mixed-effects model. Among 5804 participants who had FGF23 measurements at both visits 3 and 5, we explored predictors of FGF23 change from midlife to late life using linear regression models. Prespecified risk factors were estimated glomerular filtration rate, body mass index, ever smoking, ever drinker, diabetes, hypertension, history of cardiovascular disease, total cholesterol, and high-density lipoprotein cholesterol.

Results

Median FGF23 concentrations were 41.9 pg/mL (interquartile interval [IQI], 33.9 to 51.8 pg/mL) at visit 2, 38.3 pg/mL (IQI, 30.6 to 48.3 pg/mL) at visit 3, and 55.0 pg/mL (IQI, 44.4 to 70.3 pg/mL) at visit 5. A linear mixed-effects model showed that the association of FGF23 with age was nonlinear, with a slight decline or no change in age 45-60 years and a monotonic increase in age greater than or equal to 65 years (FGF23, +10 to 15 pg/mL per 10 years of age). In a multivariable linear regression model, significantly greater increases in FGF23 were noted, with midlife estimated glomerular filtration rate less than 60 mL/min per 1.73 m2 vs more than or equal to 60 mL/min per 1.73 m2 (ΔFGF23, +4.4 pg/mL [95% CI, 0.9 to 8.0]), diabetes vs no diabetes (ΔFGF23, +6.2 pg/mL [95% CI, 4.1 to 8.3]), and hypertension vs no hypertension (ΔFGF23, +4.1 pg/mL [95% CI, 2.7 to 5.4]).

Conclusion

FGF23 did not show any major changes in midlife but increased linearly in late life. Reduced kidney function, diabetes, and hypertension were robustly associated with a greater increase in FGF23. Further investigations are needed to understand the potential mechanisms linking these conditions to an increase in FGF23 concentrations.

Infigratinib in Patients with Recurrent Gliomas and FGFR Alterations: A Multicenter Phase II Study

PURPOSE

FGFR genomic alterations (amplification, mutations, and/or fusions) occur in ∼8% of gliomas, particularly FGFR1 and FGFR3. We conducted a multicenter open-label, single-arm, phase II study of a selective FGFR1-3 inhibitor, infigratinib (BGJ398), in patients with FGFR-altered recurrent gliomas.

PATIENTS AND METHODS

Adults with recurrent/progressive gliomas harboring FGFR alterations received oral infigratinib 125 mg on days 1 to 21 of 28-day cycles. The primary endpoint was investigator-assessed 6-month progression-free survival (PFS) rate by Response Assessment in Neuro-Oncology criteria. Comprehensive genomic profiling was performed on available pretreatment archival tissue to explore additional molecular correlations with efficacy.

RESULTS

Among 26 patients, the 6-month PFS rate was 16.0% [95% confidence interval (CI), 5.0-32.5], median PFS was 1.7 months (95% CI, 1.1-2.8), and objective response rate was 3.8%. However, 4 patients had durable disease control lasting longer than 1 year. Among these, 3 had tumors harboring activating point mutations at analogous positions of FGFR1 (K656E; n = 2) or FGFR3 (K650E; n = 1) in pretreatment tissue; an FGFR3-TACC3 fusion was detected in the other. Hyperphosphatemia was the most frequently reported treatment-related adverse event (all-grade, 76.9%; grade 3, 3.8%) and is a known on-target toxicity of FGFR inhibitors.

CONCLUSIONS

FGFR inhibitor monotherapy with infigratinib had limited efficacy in a population of patients with recurrent gliomas and different FGFR genetic alterations, but durable disease control lasting more than 1 year was observed in patients with tumors harboring FGFR1 or FGFR3 point mutations or FGFR3-TACC3 fusions. A follow-up study with refined biomarker inclusion criteria and centralized FGFR testing is warranted.

Fibroblast growth factor receptor signalling dysregulation and targeting in breast cancer

Fibroblast Growth Factor Receptor (FGFR) signalling plays a critical role in breast embryonal development, tissue homeostasis, tumorigenesis and metastasis. FGFR, its numerous FGF ligands and signalling partners are often dysregulated in breast cancer progression and are one of the causes of resistance to treatment in breast cancer. Furthermore, FGFR signalling on epithelial cells is affected by signals from the breast microenvironment, therefore increasing the possibility of breast developmental abnormalities or cancer progression. Increasing our understanding of the multi-layered roles of the complex family of FGFRs, their ligands FGFs and their regulatory partners may offer novel treatment strategies for breast cancer patients, as a single agent or rational co-target, which will be explored in depth in this review.

Muscle and Bone Impairment in Infantile Nephropathic Cystinosis: New Concepts

Cystinosis Metabolic Bone Disease (CMBD) has emerged during the last decade as a well-recognized, long-term complication in patients suffering from infantile nephropathic cystinosis (INC), resulting in significant morbidity and impaired quality of life in teenagers and adults with INC. Its underlying pathophysiology is complex and multifactorial, associating complementary, albeit distinct entities, in addition to ordinary mineral and bone disorders observed in other types of chronic kidney disease. Amongst these long-term consequences are renal Fanconi syndrome, hypophosphatemic rickets, malnutrition, hormonal abnormalities, muscular impairment, and intrinsic cellular bone defects in bone cells, due to CTNS mutations. Recent research data in the field have demonstrated abnormal mineral regulation, intrinsic bone defects, cysteamine toxicity, muscle wasting and, likely interleukin-1-driven inflammation in the setting of CMBD. Here we summarize these new pathophysiological deregulations and discuss the crucial interplay between bone and muscle in INC. In future, vitamin D and/or biotherapies targeting the IL1β pathway may improve muscle wasting and subsequently CMBD, but this remains to be proven.

The synthesis of fibroblast growth factor 23 is upregulated by homocysteine in UMR106 osteoblast-like cells

OBJECTIVES

Fibroblast growth factor 23 (FGF23) controls the production and degradation of biologically active vitamin D, 1,25(OH)₂D₃, and phosphate reabsorption in the kidney as a hormone synthesized by bone cells. Additional paracrine effects in other organs exist as well. As a biomarker, the FGF23 plasma concentration increases in renal and cardiovascular diseases, and is correlated with outcome. The regulation of FGF23 is incompletely understood and dependent on several factors, including oxidative stress. L-homocysteine is an amino acid produced in methionine metabolism, and can be converted into further metabolites depending on the availability of vitamin B. Hyperhomocysteinemia is a potential cardiovascular risk factor. Our study aimed to explore whether homocysteine impacts FGF23 synthesis.

METHODS

Experiments were performed in UMR106 osteoblast-like cells. Fgf23 gene expression and FGF23 protein concentration were measured by quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. Oxidative stress was determined by 2',7'-dichlorofluorescein diacetate fluorescence.

RESULTS

Homocysteine dose-dependently upregulated Fgf23 gene expression and protein synthesis. Moreover, homocysteine imposed oxidative stress on UMR106 cells. The effect of homocysteine on Fgf23 was abrogated by antioxidant ascorbic acid.

CONCLUSIONS

Homocysteine is a potent stimulator of FGF23 production, an effect at least in part mediated by oxidative stress. The homocysteine-dependent upregulation of FGF23 presumably contributes to its role as a cardiovascular risk factor.

Targeted therapies: Expanding the role of FGFR3 inhibition in urothelial carcinoma

The management of urothelial carcinoma (UC) has rapidly advanced in recent years with new approvals for immune checkpoint inhibitors and antibody-drug conjugates. However, while many UC tumors contain potentially actionable mutations, the role for targeted small molecule inhibitors has been limited. One such target is the fibroblast growth factor receptor (FGFR) family of proteins. Activating mutations and amplifications of FGFR3 are common in UC with higher incidences seen in upper tract as compared to lower tract disease. Consequently, multiple FGFR-directed targeted therapies have been developed and trialed in both UC and other solid tumors harboring FGFR mutations. At current, erdafitinib, an inhibitor of FGFR1-4, is the only approved targeted therapy in metastatic UC following the BLC2001 study, which demonstrated a 49% overall response rate in patients with UC harboring an FGFR3 mutation. Additional FGFR-directed agents also continue to be investigated across multiple disease stages in FGFR-mutated UC including infigratinib, rogaratinib, and AZD4547, among others. Ongoing trials are combining these agents with immune checkpoint inhibitors and chemotherapy regimens. The precision medicine revolution has begun in UC, and FGFR3 inhibitors are leading the charge toward a more personalized, biomarker-driven treatment paradigm.

Selective FGFR/FGF pathway inhibitors: inhibition strategies, clinical activities, resistance mutations, and future directions

Introduction: Fibroblast growth factor receptor (FGFR)/fibroblast growth factor (FGF) is a pathway characterized by recurring alterations in cancer. Its dysregulations enhance cancer cell proliferation, survival, migration and invasion, as well as angiogenesis and immune evasion.Areas covered: FGFR/FGF selective inhibitors belong to a broad class of drugs with some being approved for specific indications and others under investigation in ongoing phase I and II clinical trials. In this review, all available clinical data from trials on selective FGFR/FGF inhibitors as well as described resistance mutations and mechanisms are presented. FGFR/FGF pathway inhibitors are classified according to the mechanism they employ to dampen/suppress signaling and to the preferred FGFR binding mode when X-ray crystal structure is available.Expert opinion: Data presented suggests the general actionability of FGFR1,2,3 mutations and fusions across histologies, whereas FGFR1,2,3 amplifications alone are poor predictors of response to tyrosine kinase inhibitors. Overexpression on immunohistochemistry (IHC) of FGF19, the stimulatory ligand of FGFR4, can predict response to FGFR selective inhibitors in hepatocellular carcinoma. Whereas IHC overexpression of FGFR1,2,3 is not sufficient to predict benefit from FGFR inhibitors across solid tumors. FGFR1,2,3 mRNA overexpression can predict response even in absence of structural alteration. Data on resistance mutations suggests the need for new inhibitors to overcome gatekeeper mutations.

Discovery of a Potent Degrader for Fibroblast Growth Factor Receptor 1/2

Aberrant activation of FGFR signaling occurs in many cancers, and ATP-competitive FGFR inhibitors have received regulatory approval. Despite demonstrating clinical efficacy, these inhibitors exhibit dose-limiting toxicity, potentially due to a lack of selectivity amongst the FGFR family and are poorly tolerated. Here, we report the discovery and characterization of DGY-09-192, a bivalent degrader that couples the pan-FGFR inhibitor BGJ398 to a CRL2VHL E3 ligase recruiting ligand, which preferentially induces FGFR1&2 degradation while largely sparing FGFR3&4. DGY-09-192 exhibited two-digit nanomolar DC50 s for both wildtype FGFR2 and several FGFR2-fusions, resulting in degradation-dependent antiproliferative activity in representative gastric cancer and cholangiocarcinoma cells. Importantly, DGY-09-192 induced degradation of a clinically relevant FGFR2 fusion protein in a xenograft model. Taken together, we demonstrate that DGY-09-192 has potential as a prototype FGFR degrader.

Excretion of urine extracellular vesicles bearing markers of activated immune cells and calcium/phosphorus physiology differ between calcium kidney stone formers and non-stone formers

BACKGROUNDS

Previous studies have demonstrated that excretion of urinary extracellular vesicles (EVs) from different nephron segments differs between kidney stone formers and non-stone formers (NSFs), and could reflect pathogenic mechanisms of urinary stone disease. In this study we quantified selected populations of specific urinary EVs carrying protein markers of immune cells and calcium/phosphorus physiology in calcium oxalate stone formers (CSFs) compared to non-stone formers (NSFs).

METHODS

Biobanked urine samples from CSFs (n = 24) undergoing stone removal surgery and age- and sex- matched NSFs (n = 21) were studied. Urinary EVs carrying proteins related to renal calcium/phosphorus physiology (phosphorus transporters (PiT1 and PiT2), Klotho, and fibroblast growth factor 23 (FGF23); markers associated with EV generation (anoctamin-4 (ANO4) and Huntington interacting protein 1 (HIP1)), and markers shed from activated immune cells were quantified by standardized and published method of digital flow cytometry.

RESULTS

Urine excretion of calcium, oxalate, phosphorus, and calcium oxalate supersaturation (SS) were significantly higher in CSFs compared to NSFs (P < 0.05). Urinary excretion of EVs with markers of total leukocytes (CD45), neutrophils (CD15), macrophages (CD68), Klotho, FGF23, PiT1, PiT2, and ANO4 were each markedly lower in CSFs than NSFs (P < 0.05) whereas excretion of those with markers of monocytes (CD14), T-Lymphocytes (CD3), B-Lymphocytes (CD19), plasma cells (CD138 plus CD319 positive) were not different between the groups. Urinary excretion of EVs expressing PiT1 and PiT2 negatively (P < 0.05) correlated with urinary phosphorus excretion, whereas excretion of EVs expressing FGF23 negatively (P < 0.05) correlated with both urinary calcium and phosphorus excretion. Urinary EVs with markers of HIP1 and ANO4 correlated negatively (P < 0.05) with clinical stone events and basement membrane calcifications on papillary tip biopsies.

CONCLUSIONS

Urinary excretion of EVs derived from specific types of activated immune cells and EVs with proteins related to calcium/phosphorus regulation differed between CSFs and NSFs. Further validation of these and other populations of urinary EVs in larger cohort could identify biomarkers that elucidate novel pathogenic mechanisms of calcium stone formation in specific subsets of patients.

The bone is the major source of high circulating intact fibroblast growth factor-23 in acute murine polymicrobial sepsis induced by cecum ligation puncture

Fibroblast growth factor-23 (FGF23), a bone-produced hormone, plays a critical role in mineral homeostasis. Human diseases associated with excessive intact circulating FGF23 (iFGF23) result in hypophosphatemia and low vitamin D hormone in patients with normal kidney function. In addition, there is accumulating evidence linking FGF23 with inflammation. Based on these studies and the frequent observation of hypophosphatemia among septic patients, we sought to elucidate further the relationship between FGF23 and mineral homeostasis in a clinically relevant murine polymicrobial sepsis model. Medium-severity sepsis was induced by cecum ligation puncture (CLP) in adult CD-1 mice of both sexes. Healthy CD-1 mice (without CLP) were used as controls. Forty-eight hours post-CLP, spontaneous urine was collected, and serum, organs and bones were sampled at necropsy. Serum iFGF23 increased ~20-fold in CLP compared to control mice. FGF23 protein concentration was increased in the bones, but not in spleen or liver of CLP mice. Despite the ~20-fold iFGF23 increase, we did not observe any significant changes in mineral homeostasis or parathyroid hormone levels in the blood of CLP animals. Urinary excretion of phosphate, calcium, and sodium remained unchanged in male CLP mice, whereas female CLP mice exhibited lower urinary calcium excretion, relative to healthy controls. In line with renal FGF23 resistance, expression of phosphate-, calcium- and sodium-transporting proteins did not show consistent changes in the kidneys of male and female CLP mice. Renal expression of the co-receptor αKlotho was downregulated in female, but not in male CLP mice. In conclusion, our data demonstrate that the dramatic, sex-independent rise in serum iFGF23 post-CLP was mainly caused by an upregulation of FGF23 secretion in the bone. Surprisingly, the upsurge in circulating iFGF23 did not alter humoral mineral homeostasis in the acutely septic mice. Hence, the biological function of elevated FGF23 in sepsis remains unclear and warrants further studies.

Excessive fibroblast growth factor 23 promotes renal fibrosis in mice with type 2 cardiorenal syndrome

Cardiorenal syndrome (CRS) has a high mortality, but its pathogenesis remains elusive. Fibroblast growth factor 23 (FGF23) is increased in both renal dysfunction and cardiac dysfunction, and FGF receptor 4 (FGFR4) has been identified as a receptor for FGF23. Deficiency of FGF23 causes growth retardation and shortens the lifespan, but it is unclear whether excess FGF23 is detrimental in CRS. This study sought to investigate whether FGF23 plays an important role in CRS-induced renal fibrosis. A mouse model of CRS was created by surgical myocardial infarction for 12 weeks. CRS mice showed a significant increase of circulatory and renal FGF23 protein levels, as well as an upregulation of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin, a downregulation of renal Klotho expression and induction of cardiorenal dysfunction and cardiorenal fibrosis. These changes were enhanced by cardiac overexpression of FGF23 and attenuated by FGF receptor blocker PD173074 or β-catenin blocker IGC001. In fibroblasts (NRK-49F), expression of FGFR4 rather than Klotho was detected. Recombinant FGF23 upregulated the expression of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin proteins. These changes were attenuated by FGFR4 blockade with BLU9931 or β-catenin blockade with IGC001. We concluded that FGF23 promotes CRS-induced renal fibrosis mediated by partly activating FGFR4/β-catenin signaling pathway.

Overcoming Resistance to Targeted Therapies in Gastrointestinal Cancers: Progress to Date and Progress to Come

Targeted therapies have transformed the treatment paradigm in diseases such as non-small cell lung cancer and melanoma but have shown relatively modest clinical benefit in gastrointestinal malignancies. Anti-EGFR therapy in RAS wild-type colorectal cancer, anti-HER2 therapy in HER2- amplified esophagogastric cancer, and FGFR and isocitrate dehydrogenase 1 (IDH1) inhibitors in FGFR2 fusion-positive and IDH1-mutant biliary tract cancers offer antitumor efficacy, but the clinical benefit and durability of response in each case are typically limited. We review targeted therapies in each of these therapeutic areas and discuss their clinical efficacy, mechanisms of primary and acquired resistance, and strategies to overcome this resistance. We discuss lessons learned that we hope will lead to an expanded role for molecularly targeted therapy options for patients with gastrointestinal cancers.

Fibroblast growth factor 23-Klotho and hypertension: experimental and clinical mechanisms

Hypertension (HTN) and chronic kidney disease (CKD) are increasingly recognized in pediatric patients and represent risk factors for cardiovascular morbidity and mortality later in life. In CKD, enhanced tubular sodium reabsorption is a leading cause of HTN due to augmented extracellular fluid volume expansion. The renin-angiotensin-aldosterone system (RAAS) upregulates various tubular sodium cotransporters that are also targets of the hormone fibroblast growth factor 23 (FGF23) and its co-receptor Klotho. FGF23 inhibits the activation of 1,25-dihydroxyvitamin D that is a potent suppressor of renin biosynthesis. Here we review the complex interactions and disturbances of the FGF23-Klotho axis, vitamin D, and the RAAS relevant to blood pressure regulation and discuss the therapeutic strategies aimed at mitigating their pathophysiologic contributions to HTN.

New Aspects of the Kidney in the Regulation of Fibroblast Growth Factor 23 (FGF23) and Mineral Homeostasis

The bone-derived hormone fibroblast growth factor 23 (FGF23) acts in concert with parathyroid hormone (PTH) and the active vitamin D metabolite calcitriol in the regulation of calcium (Ca) and phosphate (P) homeostasis. More factors are being identified to regulate FGF23 levels and the endocrine loops between the three hormones. The present review summarizes the complex regulation of FGF23 and the disturbed FGF23/Klotho system in chronic kidney disease (CKD). In addition to the reduced ability of the injured kidney to regulate plasma levels of FGF23, several CKD-related factors have been shown to stimulate FGF23 production. The high circulating FGF23 levels have detrimental effects on erythropoiesis, the cardio-vascular system and the immune system, all contributing to the disturbed system biology in CKD. Moreover, new factors secreted by the injured kidney and the uremic calcified vasculature play a role in the mineral and bone disorder in CKD and create a vicious pathological crosstalk.

The crosstalk between bone metabolism, lncRNAs, microRNAs and mRNAs in coronary artery calcification

The association between Coronary Artery Calcification (CAC) and osteoporosis has been reported but not fully understood. Therefore, using an original bioinformatic framework we analyzed transcriptomic profiles of 20 elderly women with high CAC score and 31 age- and sex-matching controls from São Paulo Ageing & Health study (SPAH). We integrated differentially expressed microRNA (miRNA) and long-noncoding RNA (lncRNA) interactions with coding genes associated with CAC, in the context of bone-metabolism genes mined from literature. Top non-coding regulators of bone metabolism in CAC included miRNA 497-5p/195 and 106a-5p, and lncRNA FAM197Y7. Top non-coding RNAs revealed significant interplay between genes regulating bone metabolism, vascularization-related processes, chromatin organization, prostaglandin and calcium co-signaling. Prostaglandin E2 receptor 3 (PTGER3), Fibroblasts Growth Factor Receptor 1 (FGFR1), and One Cut Homeobox 2 (ONECUT2) were identified as the most susceptible to regulation by the top non-coding RNAs. This study provides a flexible transcriptomic framework including non-coding regulation for biomarker-related studies.

Osteocytic FGF23 and Its Kidney Function

Osteocytes, which represent up to 95% of adult skeletal cells, are deeply embedded in bone. These cells exhibit important interactive abilities with other bone cells such as osteoblasts and osteoclasts to control skeletal formation and resorption. Beyond this local role, osteocytes can also influence the function of distant organs due to the presence of their sophisticated lacunocanalicular system, which connects osteocyte dendrites directly to the vasculature. Through these networks, osteocytes sense changes in circulating metabolites and respond by producing endocrine factors to control homeostasis. One critical function of osteocytes is to respond to increased blood phosphate and 1,25(OH)2 vitamin D (1,25D) by producing fibroblast growth factor-23 (FGF23). FGF23 acts on the kidneys through partner fibroblast growth factor receptors (FGFRs) and the co-receptor Klotho to promote phosphaturia via a downregulation of phosphate transporters, as well as the control of vitamin D metabolizing enzymes to reduce blood 1,25D. In the first part of this review, we will explore the signals involved in the positive and negative regulation of FGF23 in osteocytes. In the second portion, we will bridge bone responses with the review of current knowledge on FGF23 endocrine functions in the kidneys.

Bone and Mineral Metabolism in Children with Nephropathic Cystinosis Compared with other CKD Entities

CONTEXT

Children with nephropathic cystinosis (NC) show persistent hypophosphatemia, due to Fanconi syndrome, as well as mineral and bone disorders related to chronic kidney disease (CKD); however, systematic analyses are lacking.

OBJECTIVE

To compare biochemical parameters of bone and mineral metabolism between children with NC and controls across all stages of CKD.

DESIGN

Cross-sectional multicenter study.

SETTING

Hospital clinics.

PATIENTS

Forty-nine children with NC, 80 CKD controls of the same age and CKD stage.

MAIN OUTCOME MEASURES

Fibroblast growth factor 23 (FGF23), soluble Klotho, bone alkaline phosphatase (BAP), tartrate-resistant acid phosphatase 5b (TRAP5b), sclerostin, osteoprotegerin (OPG), biochemical parameters related to mineral metabolism, and skeletal comorbidity.

RESULTS

Despite Fanconi syndrome medication, NC patients showed an 11-fold increased risk of short stature, bone deformities, and/or requirement for skeletal surgery compared with CKD controls. This was associated with a higher frequency of risk factors such as hypophosphatemia, hypocalcemia, low parathyroid hormone (PTH), metabolic acidosis, and a specific CKD stage-dependent pattern of bone marker alterations. Pretransplant NC patients in mild to moderate CKD showed a delayed increase or lacked an increase in FGF23 and sclerostin, and increased BAP, TRAP5b, and OPG concentrations compared with CKD controls. Post-transplant, BAP and OPG returned to normal, TRAP5b further increased, whereas FGF23 and PTH were less elevated compared with CKD controls and associated with higher serum phosphate.

CONCLUSIONS

Patients with NC show more severe skeletal comorbidity associated with distinct CKD stage-dependent alterations of bone metabolism than CKD controls, suggesting impaired mineralization and increased bone resorption, which is only partially normalized after renal transplantation.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Toward a More Precise Future for Oncology

Prospective molecular characterization of cancer has enabled physicians to define the genomic changes of each patient's tumor in real time and select personalized therapies based on these detailed portraits. Despite the promise of such an approach, previously unrecognized biological and therapeutic complexity is emerging. Here, we synthesize lessons learned and discuss the steps required to extend the benefits of genome-driven oncology, including proposing strategies for improved drug design, more nuanced patient selection, and optimized use of available therapies. Finally, we suggest ways that next-generation genome-driven clinical trials can evolve to accelerate our understanding of cancer biology and improve patient outcomes.

Fibroblast growth factor 23 predicts carotid atherosclerosis in individuals without kidney disease. The CORDIOPREV study

BACKGROUND

Fibroblast growth factor 23 (FGF23) is a major determinant of mineral metabolism derangements and emerges as a possible risk factor underlying the negative cardiovascular outcome in CKD patients. However, its contribution in non-CKD individuals is less clear. This cross-sectional study investigated the associations between FGF23 and mineral metabolism parameters and with carotid atherosclerosis in a population at high cardiovascular risk with preserved renal function.

METHODS

We employed 939 subjects with coronary heart disease enrolled in the CORDIOPREV study (mean eGFR=93.0 ± 0.7 ml/min/1.73 m² and median FGF23=44.9 (IQR=13.1) pg/ml), in which intima-media thickness of both common carotid arteries (IMT-CC) was measured.

RESULTS

Adjusted for anthropometric factors, FGF23 associated positively with creatinine, phosphate, calcium and 25(OH)-vitaminD and negatively with eGFR and calcitriol. In multivariable-adjusted models all of them were independent contributors to FGF23 levels. FGF23 showed a positive relationship with IMT-CC; both the higher third and fourth quartiles associated significantly with IMT-CC (Beta= 0.135 and 0.187, respectively) and after additional adjustment for established cardiovascular risk factors and morbidities FGF23 remained as a significant contributor to IMT-CC. Logistic regression analysis confirmed its predictive ability to differentiate patients at higher atherosclerotic risk defined by an IMT-CC≥0.7 mm (OR for FGF23 quartiles 3 and 4 vs. 1: 1.860; 95%CI 1.209-2.862 and 2.114; 95%CI 1.339-3.337, respectively).

CONCLUSION

Even in the setting of a normally functioning phosphate-FGF23-calcitriol system, FGF23 independently associated with IMT-CC, a surrogate of atherosclerotic vascular dysfunction. This supports the notion of FGF23 as a predictor of cardiovascular risk independent of renal failure.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

How do we sense phosphate to regulate serum phosphate level?

Abnormal phosphate levels result in several pathological conditions such as rickets/osteomalacia and ectopic calcification indicating that there must be a system that regulates phosphate level within a narrow range. FGF23 has been shown to be an essential hormone regulating serum phosphate level. FGF23 binds to Klotho-FGF receptor complex to reduce serum phosphate level. Several reports suggested that FGF receptor is involved in the regulation of FGF23 production. It has been also shown that high extracellular phosphate can activate several intracellular signaling pathways. However, it has been unclear whether and how phosphate regulates FGF23 production in vivo. Our recent results indicate that high extracellular phosphate directly activates FGF receptor 1 and the downstream intracellular signaling enhances FGF23 production. Thus, there is a negative feedback system for the regulation of serum phosphate level involving FGF receptor and FGF23. We propose that FGF receptor works at least as one of phosphate sensors in the maintenance of serum phosphate level.

FGF23 induced left ventricular hypertrophy mediated by FGFR4 signaling in the myocardium is attenuated by soluble Klotho in mice

There is controversy regarding whether excess FGF23 causes left ventricular hypertrophy (LVH) directly through activation of fibroblast growth factor receptor 4 (FGFR4) in cardiomyocytes or indirectly through reductions in soluble Klotho (sK). We investigated the respective roles of myocardial FGFR4 and sKL in mediating FGF23-induced LVH using mouse genetic and pharmacological approaches. To investigate a direct role of myocardial FGFR4 in mediating the cardiotoxic effects of excess circulating FGF23, we administered rFGF23 to mice with cardiac-specific loss of FGFR4 (FGFR4 heart-cKO). We tested a model of sKL deficiency, hypertension and LVH created by the conditional deletion of FGFR1 in the renal distal tubule (FGFR1DT cKO mice). The cardioprotective effects of sKL in both mouse models was assessed by the systemic administration of recombinant sKL. We confirmed that FGF23 treatment activates PLCγ in the heart and induces LVH in the absence of membrane α-Klotho. Conditional deletion of FGFR4 in the myocardium prevented rFGF23-induced LVH in mice, establishing direct cardiotoxicity of FGF23 through activation of FGFR4. Recombinant sKL administration prevented LVH, but not HTN, in FGFR1DT cKO mice, consistent with direct cardioprotective effects. Co-administration of recombinant sKL with FGF23 in culture inhibited rFGF23-induced p-PLCγ signaling. Thus, FGF23 ability to include LVH represents a balance between FGF23 direct cardiac activation of FGFR4 and the modulating effects of circulating sKL to alter FGF23-dependent myocardial signaling pathways.

Properties of FDA-approved small molecule protein kinase inhibitors: A 2020 update

Because genetic alterations including mutations, overexpression, translocations, and dysregulation of protein kinases are involved in the pathogenesis of many illnesses, this enzyme family is currently the subject of many drug discovery programs in the pharmaceutical industry. The US FDA approved four small molecule protein kinase antagonists in 2019; these include entrectinib, erdafitinib, pexidartinib, and fedratinib. Entrectinib binds to TRKA/B/C and ROS1 and is prescribed for the treatment of solid tumors with NTRK fusion proteins and for ROS1-postive non-small cell lung cancers. Erdafitinib inhibits fibroblast growth factor receptors 1-4 and is used in the treatment of urothelial bladder cancers. Pexidartinib is a CSF1R antagonist that is prescribed for the treatment of tenosynovial giant cell tumors. Fedratinib blocks JAK2 and is used in the treatment of myelofibrosis. Overall, the US FDA has approved 52 small molecule protein kinase inhibitors, nearly all of which are orally effective with the exceptions of temsirolimus (which is given intravenously) and netarsudil (an eye drop). Of the 52 approved drugs, eleven inhibit protein-serine/threonine protein kinases, two are directed against dual specificity protein kinases, eleven target non-receptor protein-tyrosine kinases, and 28 block receptor protein-tyrosine kinases. The data indicate that 46 of these drugs are used in the treatment of neoplastic diseases (eight against non-solid tumors such as leukemias and 41 against solid tumors including breast and lung cancers; some drugs are used against both tumor types). Eight drugs are employed in the treatment of non-malignancies: fedratinib, myelofibrosis; ruxolitinib, myelofibrosis and polycythemia vera; fostamatinib, chronic immune thrombocytopenia; baricitinib, rheumatoid arthritis; sirolimus, renal graft vs. host disease; nintedanib, idiopathic pulmonary fibrosis; netarsudil, glaucoma; and tofacitinib, rheumatoid arthritis, Crohn disease, and ulcerative colitis. Moreover, sirolimus and ibrutinib are used for the treatment of both neoplastic and non-neoplastic diseases. Entrectinib and larotrectinib are tissue-agnostic anti-cancer small molecule protein kinase inhibitors. These drugs are prescribed for the treatment of any solid cancer harboring NTRK1/2/3 fusion proteins regardless of the organ, tissue, anatomical location, or histology type. Of the 52 approved drugs, seventeen are used in the treatment of more than one disease. Imatinib, for example, is approved for the treatment of eight disparate disorders. The most common drug targets of the approved pharmaceuticals include BCR-Abl, B-Raf, vascular endothelial growth factor receptors (VEGFR), epidermal growth factor receptors (EGFR), and ALK. Most of the approved small molecule protein kinase antagonists (49) bind to the protein kinase domain and six of them bind covalently. In contrast, everolimus, temsirolimus, and sirolimus are larger molecules (MW ≈ 1000) that bind to FK506 binding protein-12 (FKBP-12) to generate a complex that inhibits the mammalian target of rapamycin (mTOR) protein kinase complex. This review presents the physicochemical properties of all of the FDA-approved small molecule protein kinase inhibitors. Twenty-two of the 52 drugs have molecular weights greater than 500, exceeding a Lipinski rule of five criterion. Excluding the macrolides (everolimus, sirolimus, temsirolimus), the average molecular weight of the approved drugs is 480 with a range of 306 (ruxolitinib) to 615 (trametinib). More than half of the antagonists (29) have lipophilic efficiency values of less than five while the recommended optima range from 5 to 10. One of the troublesome problems with both targeted and cytotoxic drugs in the treatment of malignant diseases is the near universal development of resistance to every therapeutic modality.

FGF23 expression is stimulated in transgenic α-Klotho longevity mouse model

Observations in transgenic α-Klotho (Kl) mice (KlTg) defined the antiaging role of soluble Klotho (sKL130). A genetic translocation that elevates sKL levels in humans is paradoxically associated with increased circulating fibroblast growth factor 23 (FGF23) levels and the potential of both membrane KL (mKL135) and sKL130 to act as coreceptors for FGF23 activation of fibroblast growth factor receptors (FGFRs). Neither FGF23 expression nor the contributions of FGF23, mKL135, and sKL130 codependent and independent functions have been investigated in KlTg mice. In the current study, we examined the effects of Kl overexpression on FGF23 levels and functions in KlTg mice. We found that mKL135 but not sKL130 stimulated FGF23 expression in osteoblasts, leading to elevated Fgf23 bone expression and circulating levels in KlTg mice. Elevated FGF23 suppressed 1,25(OH)2D and parathyroid hormone levels but did not cause hypophosphatemic rickets in KlTg mice. KlTg mice developed low aldosterone-associated hypertension but not left ventricular hypertrophy. Mechanistically, we found that mKL135 and sKL130 are essential cofactors for FGF23-mediated ERK activation but that they inhibited FGF23 stimulation of PLC-γ and PI3K/AKT signaling. Thus, increased longevity in KlTg mice occurs in the presence of excess FGF23 that interacts with mKL and sKL to bias FGFR pathways.

The rs1256328 (ALPL) and rs12654812 (RGS14) Polymorphisms are Associated with Susceptibility to Calcium Nephrolithiasis in a Taiwanese population

Nephrolithiasis is a common disease affecting almost all populations, with an increasing prevalence over the past decades. Previous studies revealed several functional polymorphisms associated with the pathogenesis of nephrolithiasis. However, data on Asian populations are limited. In this study, three candidate polymorphisms were selected from previous studies to investigate the correlations with nephrolithiasis in a Taiwanese population. In total, 454 nephrolithiasis patients were recruited from Kaohsiung Medical University Hospital, with SNP frequency for 1513 subjects of general population from the Taiwan Biobank (TWB) as a genotypic reference. Results revealed that subjects with minor TT genotype at rs1256328 (alkaline phosphatase, liver/bone/kidney (ALPL)) have higher susceptibility to nephrolithiasis (odds ratio (OR) = 2.03, p = 0.0013). In addition, subjects carrying the minor AA genotype at rs12654812 (regulator of G protein signaling 14 (RGS14)) have higher susceptibility to nephrolithiasis (OR = 1.91, p = 0.0017). Among nephrolithiasis patients, subjects with GG at rs7627468 (calcium-sensing receptor (CASR)) have lower pH level in urine (p = 0.0088). Importantly, rs7627468 is associated with the expressions of IQCB1 and EAF2. rs12654812 could influence the expression of RGS14 itself, MXD3, and FGFR4. In summary, this study successfully validated the genetic roles of rs1256328 and rs12654812 in human nephrolithiasis.

Factors inhibiting intestinal calcium absorption: hormones and luminal factors that prevent excessive calcium uptake

Besides the two canonical calciotropic hormones, namely parathyroid hormone and 1,25-dihydroxyvitamin D [1,25(OH)2D3], there are several other endocrine and paracrine factors, such as prolactin, estrogen, and insulin-like growth factor that have been known to directly stimulate intestinal calcium absorption. Generally, to maintain an optimal plasma calcium level, these positive regulators enhance calcium absorption, which is indirectly counterbalanced by a long-loop negative feedback mechanism, i.e., through calcium-sensing receptor in the parathyroid chief cells. However, several lines of recent evidence have revealed the presence of calcium absorption inhibitors present in the intestinal lumen and extracellular fluid in close vicinity to enterocytes, which could also directly compromise calcium absorption. For example, luminal iron, circulating fibroblast growth factor (FGF)-23, and stanniocalcin can decrease calcium absorption, thereby preventing excessive calcium uptake under certain conditions. Interestingly, the intestinal epithelial cells themselves could lower their rate of calcium uptake after exposure to high luminal calcium concentration, suggesting a presence of an ultra-short negative feedback loop independent of systemic hormones. The existence of neural regulation is also plausible but this requires more supporting evidence. In the present review, we elaborate on the physiological significance of these negative feedback regulators of calcium absorption, and provide evidence to show how our body can efficiently restrict a flood of calcium influx in order to maintain calcium homeostasis.

Rogaratinib in patients with advanced cancers selected by FGFR mRNA expression: a phase 1 dose-escalation and dose-expansion study

BACKGROUND

The clinical activity of fibroblast growth factor receptor (FGFR) inhibitors seems restricted to cancers harbouring rare FGFR genetic aberrations. In preclinical studies, high tumour FGFR mRNA expression predicted response to rogaratinib, an oral pan-FGFR inhibitor. We aimed to assess the safety, maximum tolerated dose, recommended phase 2 dose, pharmacokinetics, and preliminary clinical activity of rogaratinib.

METHODS

We did a phase 1 dose-escalation and dose-expansion study of rogaratinib in adults with advanced cancers at 22 sites in Germany, Switzerland, South Korea, Singapore, Spain, and France. Eligible patients were aged 18 years or older, and were ineligible for standard therapy, with an Eastern Cooperative Oncology Group performance status of 0-2, a life expectancy of at least 3 months, and at least one measurable or evaluable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. During dose escalation, rogaratinib was administered orally twice daily at 50-800 mg in continuous 21-day cycles using a model-based dose-response analysis (continuous reassessment method). In the dose-expansion phase, all patients provided an archival formalin-fixed paraffin-embedded (FFPE) tumour biopsy or consented to a new biopsy at screening for the analysis of FGFR1-3 mRNA expression. In the dose-expansion phase, rogaratinib was given at the recommended dose for expansion to patients in four cohorts: urothelial carcinoma, head and neck squamous-cell cancer (HNSCC), non-small-cell lung cancer (NSCLC), and other solid tumour types. Primary endpoints were safety and tolerability, determination of maximum tolerated dose including dose-limiting toxicities and determination of recommended phase 2 dose, and pharmacokinetics of rogaratinib. Safety analyses were reported in all patients who received at least one dose of rogaratinib. Patients who completed cycle 1 or discontinued during cycle 1 due to an adverse event or dose-limiting toxicity were included in the evaluation of recommended phase 2 dose. Efficacy analyses were reported for all patients who received at least one dose of study drug and who had available post-baseline efficacy data. This ongoing study is registered with ClinicalTrials.gov, number NCT01976741, and is fully recruited.

FINDINGS

Between Dec 30, 2013, and July 5, 2017, 866 patients were screened for FGFR mRNA expression, of whom 126 patients were treated (23 FGFR mRNA-unselected patients in the dose-escalation phase and 103 patients with FGFR mRNA-overexpressing tumours [52 patients with urothelial carcinoma, eight patients with HNSCC, 20 patients with NSCLC, and 23 patients with other tumour types] in the dose-expansion phase). No dose-limiting toxicities were reported and the maximum tolerated dose was not reached; 800 mg twice daily was established as the recommended phase 2 dose and was selected for the dose-expansion phase. The most common adverse events of any grade were hyperphosphataemia (in 77 [61%] of 126 patients), diarrhoea (in 65 [52%]), and decreased appetite (in 48 [38%]); and the most common grade 3-4 adverse events were fatigue (in 11 [9%] of 126 patients) and asymptomatic increased lipase (in 10 [8%]). Serious treatment-related adverse events were reported in five patients (decreased appetite and diarrhoea in one patient with urothelial carcinoma, and acute kidney injury [NSCLC], hypoglycaemia [other solid tumours], retinopathy [urothelial carcinoma], and vomiting [urothelial carcinoma] in one patient each); no treatment-related deaths occurred. Median follow-up after cessation of treatment was 32 days (IQR 25-36 days). In the expansion cohorts, 15 (15%; 95% CI 8·6-23·5) out of 100 evaluable patients achieved an objective response, with responses recorded in all four expansion cohorts (12 in the urothelial carcinoma cohort and one in each of the other three cohorts), and in ten (67%) of 15 FGFR mRNA-overexpressing tumours without apparent FGFR genetic aberration.

INTERPRETATION

Rogaratinib was well tolerated and clinically active against several types of cancer. Selection by FGFR mRNA expression could be a useful additional biomarker to identify a broader patient population who could be eligible for FGFR inhibitor treatment.

FUNDING

Bayer AG.

Metastatic prostate cancer presenting as tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), or oncogenic osteomalacia, is a paraneoplastic syndrome marked by hypophosphataemia, renal phosphate wasting, bone pain, weakness, and fractures. The syndrome has been reported with both benign and malignant tumours including parotid gland basal cell tumours, thyroid carcinomas, colon adenocarcinomas, and prostate cancer. Often, the syndrome is marked by an insidious course during which patients present with generalised bony pain and weakness, which do not resolve until the underlying tumour is identified and treated. We present a case of a patient with Parkinson's disease whose subacute weakness, lower extremity paresis, and renal phosphate wasting led to the synchronous diagnosis of metastatic prostate adenocarcinoma and TIO.

Nintedanib plus letrozole in early breast cancer: a phase 0/I pharmacodynamic, pharmacokinetic, and safety clinical trial of combined FGFR1 and aromatase inhibition

BACKGROUND

The combined use of a FGFR1 blocker and aromatase inhibitors is appealing for treating breast cancer patients with FGFR1 amplification. However, no pharmacodynamic studies have addressed the effects of this combined target modulation. We conducted a phase 0/I clinical trial in an adjuvant setting, with the goal of obtaining pharmacodynamic proof of the effects of combined aromatase and FGFR1 inhibition and to establish the RP2D for nintedanib combined with letrozole.

PATIENTS AND METHODS

Women with early-stage luminal breast cancer were eligible for enrollment in the study. Dose level 1 was nintedanib (150 mg/bid) plus letrozole (2.5 mg/day) administered for a single 28-day cycle (DLT assessment period), followed by a classic 3 + 3 schedule. FGF23 and 17-B-estradiol levels were determined on days 0 and 15; pharmacokinetic parameters were assessed on days 1 and 28. Patients were allowed to continue treatment for 6 cycles. The primary study endpoint was a demonstration of FGFR1 modulation (defined as a 25% increase in the plasma FGF23 level).

RESULTS

A total of 19 patients were enrolled in the study (10 in the expansion cohort following dose escalation). At the RP2D (nintedanib 200 mg/bid plus letrozole 2.5 mg/day), we observed a 55% mean increase in the plasma FGF23 level, and 81.2% of the patients had no detectable level of 17-B-estradiol in their plasma (87.5% of the patients treated with letrozole alone). Nintedanib and letrozole displayed a pharmacokinetic interaction that led to three- and twofold increases in their respective plasma concentrations. Most G3 toxic events (5 out of 6: 2 diarrhea and 3 hypertransaminasemia) occurred subsequent to the DLT assessment period.

CONCLUSION

Combined treatment with nintedanib (200 mg/bid) plus letrozole (2.5 mg/day) effectively suppressed FGFR1 and aromatase activity, and these respective doses can be used as starting doses in any subsequent trials. However, drug-drug interactions may produce tolerability issues when these drugs are co-administered for an extended time period (e.g., 6 months). Patients enrolled in future trials with these drugs should be carefully monitored for their FGF23 levels and signs of toxicity, and those findings should guide individualized treatment decisions.

TRIAL REGISTRATION

This trial was registered at www.clinicaltrials.gov under reg. # NCT02619162, on December 2, 2015.

Effects of etelcalcetide on fibroblast growth factor 23 in patients with secondary hyperparathyroidism receiving hemodialysis

Background

Etelcalcetide is an intravenous calcimimetic approved for treatment of secondary hyperparathyroidism (sHPT) in patients receiving hemodialysis. Besides lowering parathyroid hormone (PTH), etelcalcetide also significantly reduces fibroblast growth factor 23 (FGF23), but the mechanisms are unknown.

Methods

To investigate potential mediators of etelcalcetide-induced FGF23 reduction, we performed secondary analyses of the 26-week randomized trials that compared the effects on PTH of etelcalcetide (n = 509) versus placebo (n = 514) and etelcalcetide (n = 340) versus cinacalcet (n = 343) in adults with sHPT receiving hemodialysis. We analyzed changes in FGF23 in relation to changes in PTH, calcium, phosphate and bone turnover markers. We also investigated how concomitant treatments aimed at mitigating hypocalcemia altered the FGF23-lowering effects of etelcalcetide.

Results

Etelcalcetide reduced FGF23 [median % change (quartile 1–quartile 3)] from baseline to the end of the trial significantly more than placebo [–56% (–85 to –7) versus +2% (–40 to +65); P < 0.001] and cinacalcet [–68% (–87 to –26) versus –41% (–76 to +25); P < 0.001]. Reductions in FGF23 correlated strongly with reductions in calcium and phosphate, but not with PTH; correlations with bone turnover markers were inconsistent and of borderline significance. Increases in concomitant vitamin D administration partially attenuated the FGF23-lowering effect of etelcalcetide, but increased dialysate calcium concentration versus no increase and increased dose of calcium supplementation versus no increase did not attenuate the FGF23-lowering effects of etelcalcetide.

Conclusion

These data suggest that etelcalcetide potently lowers FGF23 in patients with sHPT receiving hemodialysis and that the effect remains detectable among patients who receive concomitant treatments aimed at mitigating treatment-associated decreases in serum calcium.

FGF23 Synthesis and Activity

PURPOSE OF REVIEW

The phosphaturic hormone FGF23 is produced primarily in osteoblasts/osteocytes and is known to respond to increases in serum phosphate and 1,25(OH)2 vitamin D (1,25D). Novel regulators of FGF23 were recently identified, and may help explain the pathophysiologies of several diseases. This review will focus on recent studies examining the synthesis and actions of FGF23.

RECENT FINDINGS

The synthesis of FGF23 in response to 1,25D is similar to other steroid hormone targets, but the cellular responses to phosphate remain largely unknown. The activity of intracellular processing genes control FGF23 glycosylation and phosphorylation, providing critical functions in determining the serum levels of bioactive FGF23. The actions of FGF23 largely occur through its co-receptor αKlotho (KL) under normal circumstances, but FGF23 has KL-independent activity during situations of high concentrations.

SUMMARY

Recent work regarding FGF23 synthesis and bioactivity, as well as considerations for diseases of altered phosphate balance will be reviewed.

Thyroid dysfunction from inhibitor of fibroblast growth factor receptor

Background

Thyroid dysfunction has not been previously reported in clinical trials of selective fibroblast growth factor receptor (FGFR) inhibitors including AZD4547. Herein, we report a case of worsening hypothyroidism in a patient with advanced urothelial cancer treated with AZD4547.

Case presentation

An 80-year-old Caucasian female with metastatic urothelial carcinoma failed first-line chemotherapy with gemcitabine and carboplatin and second-line treatment with atezolizumab, an inhibitor of programmed cell death ligand 1. She developed hypothyroidism at completion of atezolizumab treatment and responded to levothyroxine. Subsequently she was enrolled to a phase II study and received AZD4547 due to an actionable mutation at FGFR3 found in tumor biopsy. Two months later, she experienced recurrent hypothyroidism symptoms, and was hospitalized twice for small bowel obstruction. Her thyroid stimulating hormone level was significantly increased to 2957 uIU/mL (reference range 0.8-7.7 uIU/mL). Her levothyroxine dose was increased accordingly. Her thyroid function returned to normal 1 month afterwards, and small bowel obstruction did not recur.

Conclusion

Further reports and studies will be needed to confirm the relationship between AZD4547 and hypothyroidism. Based on this observation and possible mechanisms for thyroid dysfunction discussed in this paper, routine thyroid function monitoring in patients receiving FGFR inhibitor should be considered.